The present invention relates to chemical mechanical polishing (CMP), and more particularly, to optical endpoint detection during a CMP process.
Chemical mechanical polishing (CMP) has emerged as a crucial semiconductor technology, particularly for devices with critical dimensions smaller than 0.5 micron. One important aspect of CMP is endpoint detection (EPD), i.e., determining during the polishing process when to terminate the polishing.
Many users prefer EPD systems that are xe2x80x9cin situ EPD systemsxe2x80x9dwhich provide EPD during the polishing process. Numerous in situ EPD methods have been proposed, but few have been successfully demonstrated in a manufacturing environment and even fewer have proved sufficiently robust for routine production use.
One group of prior art in situ EPD techniques involves the electrical measurement of changes in the capacitance, the impedance, or the conductivity of the wafer and calculating the endpoint based on an analysis of this data. To date, these particular electrically based approaches to EPD are not commercially available.
One other electrical approach that has proved production worthy is to sense changes in the friction between the wafer being polished and the polish pad. Such measurements are done by sensing changes in the motor current. These systems use a global approach, i.e., the measured signal assesses the entire wafer surface. Thus, these systems do not obtain specific data about localized regions. Further, this method works best for EPD for metal CMP because of the dissimilar coefficient of friction between the polish pad and the tungsten-titanium nitride-titanium film stack versus the polish pad and the dielectric underneath the metal. However, with advanced interconnection conductors, such as copper (Cu), the associated barrier metals, e.g., tantalum or tantalum nitride, may have a coefficient of friction that is similar to the underlying dielectric. The motor current approach relies on detecting the copper-tantalum nitride transition, then adding an overpolish time. Intrinsic process variations in the thickness and composition of the remaining film stack layer mean that the final endpoint trigger time may be less precise than is desirable.
Another group of methods uses an acoustic approach. In a first acoustic approach, an acoustic transducer generates an acoustic signal that propagates through the surface layer(s) of the wafer being polished. Some reflection occurs at the interface between the layers, and a sensor positioned to detect the reflected signals can be used to determine the thickness of the topmost layer as it is polished. In a second acoustic approach, an acoustical sensor is used to detect the acoustical signals generated during CMP. Such signals have spectral and amplitude content that evolves during the course of the polish cycle. However, to date there has been no commercially available in situ endpoint detection system using acoustic methods to determine endpoint.
Finally, the present invention falls within the group of optical EPD systems. One approach for optical EPD systems is of the type disclosed in U.S. Pat. No. 5,433,651 to Lustig et al. in which a window in the platen of a rotating CMP tool is used to sense changes in a reflected optical signal. However, the window complicates the CMP process because it presents to the wafer an inhomogeneity in the polish pad. Such a region can also accumulate slurry and polish debris.
Another approach is of the type disclosed in European application EP 0 824 995 A1, which uses a transparent window in the actual polish pad itself. A similar approach for rotational polishers is of the type disclosed in European application EP 0 738 561 A1, in which a pad with an optical window is used for EPD. In both of these approaches, various means for implementing a transparent window in a pad are discussed, but making measurements without a window was not considered. The methods and apparatuses disclosed in these patents require sensors to indicate the presence of a wafer in the field of view. Furthermore, integration times for data acquisition are constrained to the amount of time the window in the pad is under the wafer.
In another type of approach, the carrier is positioned on the edge of the platen so as to expose a portion of the wafer. A fiber optic based apparatus is used to direct light at the surface of the wafer, and spectral reflectance methods are used to analyze the signal. The drawback of this approach is that the process must be interrupted in order to position the wafer in such a way as to allow the optical signal to be gathered.
In so doing, with the wafer positioned over the edge of the platen, the wafer is subjected to edge effects associated with the edge of the polish pad going across the wafer while the remaining portion of the wafer is completely exposed. An example of this type of approach is described in PCT application WO 98/05066.
In another approach, the wafer is lifted off of the pad a small amount, and a light beam is directed between the wafer and the slurry-coated pad. The light beam is incident at a small angle so that multiple reflections occur. The irregular topography on the wafer causes scattering, but if sufficient polishing is done prior to raising the carrier, then the wafer surface will be essentially flat and there will be very little scattering due to the topography on the wafer. An example of this type of approach is disclosed in U.S. Pat. No. 5,413,941. The difficulty with this type of approach is that the normal process cycle must be interrupted to make the measurement.
Yet another approach entails monitoring absorption of particular wavelengths in the infrared spectrum of a beam incident upon the backside of a wafer being polished so that the beam passes through the wafer from the nonpolished side of the wafer. Changes in the absorption within narrow, well defined spectral windows correspond to changing thickness of specific types of films. This approach has the disadvantage that, as multiple metal layers are added to the wafer, the sensitivity of the signal decreases rapidly. One example of this type of approach is disclosed in U.S. Pat. No. 5,643,046.
The above described prior art primarily deals with the apparatus used to gather the optical data. Another important aspect of endpoint detection is an effective means for analyzing and interpreting the reflected optical data that is gathered. The present invention provides a method for accurately analyzing reflectance data to determine when an endpoint has been reached.
An apparatus is provided for use with a tool for polishing thin films on a semiconductor wafer surface that detects an endpoint of a polishing process. In one embodiment, the apparatus includes a polish pad having a through-hole, a light source, a fiber optic cable assembly, a light sensor, and a computer. The light source provides light within a predetermined bandwidth. The fiber optic cable propagates the light through the through-hole to illuminate the wafer surface during the polishing process. The light sensor receives reflected light from the surface through the fiber optic cable and generates data corresponding to the spectrum of the reflected light. The computer receives the reflected spectral data and generates an endpoint signal as a function of the reflected spectral data. In the following description and discussion the term semiconductor wafer is meant to include all workpieces that are related to electronics, such as bare wafers with films, wafers partially or fully processed for forming integrated circuits and interconnecting lines, wafers partially or fully processed for forming micro-electro-mechanical devices (MEMS), specialized circuit assembly substrates, circuit boards, hybrid circuits, hard disk platters, flat panel display substrates, or other structures that would benefit from CMP with end point detection.
In a metal film polishing application, the endpoint signal is a function of the intensities of at least two individual wavelength bands selected from the predetermined bandwidth. In a dielectric film polishing application, the endpoint signal is based upon fitting of the reflected spectrum to an optical reflectance model to determine remaining film thickness. The computer compares the endpoint signal to predetermined criteria and stops the polishing process when the endpoint signal meets the predetermined criteria. Unlike prior art optical endpoint detection systems, an apparatus according to the present invention, together with the endpoint detection methodology, advantageously allows for accuracy and reliability in the presence of accumulated slurry and polishing debris. This robustness makes the apparatus suitable for in situ EPD in a production environment.
In an alternative embodiment, endpoint detection is accomplished by a comparison between a reference spectrum and the gathered reflectance spectrum. The reference spectrum is obtained by polishing a wafer to the process of record polish time and conditions while collecting the reflectance spectra vs. time from the wafer. A reflectance spectrum at a selected time just prior to the full polish time is then selected as the reference spectrum. One or more wafers may be used to establish the reference spectrum.
For wafers with a metal film to be polished, the reference time and corresponding reference spectrum are typically selected at a time that corresponds to stable polishing of the metal film before the onset of clearing of the metal film occurs. When clearing occurs, the reflected spectrum will be substantially different from the reference spectrum taken during the metal phase. In effect, for polishing of metal films, the method answers the question of when is the spectrum not like the metal reference spectrum. Since the metal film reflectance spectrum is quite similar from wafer to wafer, the reference spectrum may be taken from a reference wafer, or it may be taken each time a wafer is polished from the wafer itself, during the metal polishing phase before any clearing takes place.
Also please note that if it is desired to generate an endpoint on a barrier film between the metal film and a dielectric layer, the reference spectrum may be taken from the barrier layer with the appropriate reference wafer.
For dielectric film wafers, where the film reflectance changes during polishing, it is preferred to take a reference spectrum near the desired end point from a reference wafer. If it is desirable to know when, for example, half of the dielectric layer has been removed, a reference spectrum should be taken from the reference wafer that corresponds to half of the film being removed. The selection of the reference spectrum corresponds to the desired information from the film being polished.
Production wafers are then polished and the reflectance spectrum is continuously gathered. A comparison is made between the reference spectrum and the reflectance spectrum. In the case of polishing dielectric layers, when the difference between the reflectance spectrum and the reference spectrum is below a predetermined threshold, then the endpoint has been reached. In the case of polishing metal layers, when the difference between the reflectance spectrum and the reference spectrum is greater than a predetermined threshold, then the endpoint has been reached.